The invention relates to a process for preparing an azo derivative with ultra fast optical response. According to the invention, the process consists in preparing saturated solutions of organic compound 1-phenyl-3-methyl-azo-(1′-carboxy phenylen)-5-one which are subjected to ultrasonication for 5 min and progressively increased each 10 min, up to 50 min, at an ultrasonication frequency of 24…27 Hz, after which the crystals of the resulting precipitate are collected and subjected to laser irradiation at a constant intensity of the absorbed laser radiation of 200 mW, until reaching a constant SHG saturation signal corresponding to the maximum molecular crystalline order.